This invention relates to a recording method of a phase change optical recording medium and recording apparatus. More particularly, the invention relates to a method and apparatus for recording and reproducing information by irradiating an optical beam onto a phase change optical recording medium, improved in erasability than conventional ones.
Optical recording mediums for recording and reproducing information by irradiation of an optical beam are available for large capacities, quick access and portability, and therefore used in storage means for computers and for video and audio files. Their further development is still being expected. Especially, phase change optical recording mediums are hopeful to be one of the leading mainstreams of rewritable optical recording mediums because data can be readily overwritten thereon by irradiation of a single beam and because they are of the reflectivity-changeable reproduction type which facilitates compatibility with CD-ROM (compact disc read only memory).
Optical disc are explained below as an example of optical recording mediums.
Phase change optical discs are configured to record and reproduce information thereon by irradiating a recording beam modulated in optical intensity, basically using three power levels, namely, a first recording power level (Pa) for making an xe2x80x9camorphous markxe2x80x9d by once heating the recording layer to its melting point or a higher temperature to change it into a molten random state and then quench it to the room temperature by rapid cooling; a second recording power level (Pc) for making a xe2x80x9ccrystal space portionxe2x80x9d by heating the recording layer to a temperature zone lower than its melting point and not lower than its crystallizing temperature and then gradually cooling it for crystallization; and a reproduction power level (Pr) for maintaining the recording medium below its crystallization temperature to read out information in a nondestructive mode.
For making an amorphous mark of a predetermined length in the recording track direction, it must be essentially sufficient to continuously irradiate light of the Pa level for a time corresponding to the length of the mark. Actually, however, continuous irradiation makes the temperature of the recording layer higher at the end of irradiation than at the start of irradiation, and results in undesirably making a droplet-shaped mark. Especially in the mark-length recording and edge detecting system which is advantageous from the viewpoint of the recording density, droplet-shaped marks cause fluctuation of edge positions of reproduced signals, and adversely affect the linear density.
Taking it into consideration, means for irradiating pulses of Pa level light has been used on conventional phase change discs, especially on phase change optical discs employing the mark length record edge detecting system. For example, a first-generation DVD-RAM (digital versatile disc-random access memory) recording pulse irradiation method using phase change discs was introduced in Nikkei Electronics, No. 1997.10.6, p.318. The use of pulses of Pa level light enabled recording with less fluctuation in mark edge positions and a high linear density.
Conventionally, however, with no particular consideration on the crystallizing level, Pc, there has been employed a technology of continuously irradiating a Pc level light beam to a portion to be crystallized for a time corresponding to its length. As disclosed on p. 318 of the same publication, Nikkei Electronics No. 1997.10.6, there is a technique of irradiating light of a power level other than the Pc level immediately after or before making an amorphous record mark among others to be crystallized. However, in almost all areas of the crystal space to be made among marks, optical beams of a predetermined intensity in the Pc level has been used. Japanese Patent Laid-Open Publication No. H 9-7176 also discloses a technique of irradiating beams in form of pulses onto portions for making record marks, but gives no teaching on irradiating beams in form of pulses onto portions for making crystal spaces.
The present invention takes the above-mentioned problems as its background. That is, through researches on various ways of irradiation of light upon making crystal spaces, the Inventor found that the use of Pc level light in form of pulses is remarkably effective for improving the erasability, and attained the present invention. The invention demonstrates its effects in all phase change optical discs, but it is particularly effective in its modes for recording from a non-initialized state, that is, in modes for recording crystal spaces in an as-depo amorphous state.
In order to improve the characteristics of a phase change optical disc and to improve, in particular, the overwrite erasability, the invention provides the recording method summarized below.
That is, according to the invention, there is provided a recording method of a phase change optical recording medium having a recording layer, comprising the steps of:
making an amorphous mark by irradiating laser light and thereby changing a part of the recording layer to an amorphous phase; and
making a crystal space portion by irradiating laser light and thereby crystallizing a part of the recording layer, the step of making the crystal space portion including: a first step of irradiating laser light of a first power level; and a second step of irradiating laser light of a second power level different from the first power level, and the recording layer being substantially changed in temperature in accordance with the first step and the second step.
In a preferable mode, at least one of the first step and the second step is executed within a time shorter than a time required for the full width at half maximum of the laser light to pass through a point of the recording layer.
Especially when the phase change optical recording medium is a non-initialized medium in which the recording layer is initially amorphous before recording, the crystal space portion can be made effectively.
According to the invention, there is further provided a recording apparatus for a phase change optical recording medium having a recording layer, comprising:
means for making an amorphous mark by irradiating laser light and thereby changing the recording layer into an amorphous phase; and
the means for making a crystal space portion by irradiating laser light and thereby crystallizing the recording layer, the means for making the crystal space portion being configured to execute a first step of irradiating laser light of a first power level, and a second step of irradiating laser light of a second power level different from the first power level.
In a preferable mode of the invention, the means for making the crystal space portion is configured to execute at least one of the first step and the second step within a time shorter than a time required for the full width at half maximum of the laser light to pass through a point of the recording layer.
In a specific example of the recording method according to the invention, for recording, erasing or reproducing information by irradiating onto a phase change optical disc those light beams of at least three power levels, namely, the first recording power level (Pa) for changing the recording layer into an amorphous phase, the second recording power level (Pc) for crystallizing the recording layer and the reproducing power level (Pr), at least the light of the second recording power level is formed into pulses between a peak level (Pc1) lower at least than the first recording power level and a bottom power level (Pc2) lower than Pc1, and crystal space portions can be made in the phase change recording layer by irradiating a row of these pulses of the light beam.
Temperature near the edge of the track in the phase change recording layer may be increased to a temperature zone promoting generation of crystal nuclei by irradiating light of the Pc1 level, and temperature near the center of the track in the phase change recording layer may be increased to a temperature zone promoting generation of crystal nuclei by irradiating light of the Pc2 level.
In the present invention, it is sufficient for the second recording power level of the erasure signal for crystallizing the recording layer to be divided into two or more steps, and it is also acceptable that the first recording power level for changing the recording layer amorphous is used in form of pulses as well, like the conventional technique. Stepping the second recording power level does not mean changing the level merely for the portion near the mark upon recording crystal spaces for the purpose of adjusting the position of the front end or rear end of an amorphous mark, but does mean the use of the pulse beam for making crystal spaces also in areas distant from the amorphous mark.
As set forth above, light of the second recording power level is formed into pulses, for example, between the peak level (Pc1) lower than the first recording power level and the bottom power level (Pc2) lower than Pc1. The upper limit of Pc1 is the power heating the recording layer to the melting point, and the lower limit of Pc2 is the power heating the temperature in the central portion of the track in the recording layer to the crystallization temperature. Within this range, Pc1 and Pc2 are determined to best meet the thermal response of the medium. By determining Pc1 and Pc2 in this manner, irradiation of light in the Pc1 level increases the temperature near the track edge in the phase change recording layer to the temperature zone promoting generation of crystal nuclei, and irradiation of light in the Pc2 level increases the temperature near the central part of the track in the phase change recording layer to the temperature zone promoting generation of crystal nuclei. As a result, the recording layer, throughout the entire area of the track width thereof, is maintained in the temperature zone promoting generation of crystal nuclei for a uniform time, and the overwrite erasability is improved.
It is when the initial status of the phase change recording layer before recording is amorphous, namely, when the disc is a non-initialized disc, that the invention is most effective. Non-initialized discs permit direct recording not through the step of initial crystallization of the as-depo amorphous status under special film-making conditions explained later. Recording on a non-initialized disc is executed by recording crystal spaces into the amorphous material rather than making amorphous marks. Therefore, the recording method according to the invention, which promotes crystallization (which, in other words, ensures a high erasability) promises the largest effect when applied to non-initialized discs.
According to the invention, by dividing the erasure signal into at least two kinds of steps upon making a crystal space, it is possible to maintain the recording film with a good balance in a temperature zone increasing the crystal nuclei generating frequency and in a temperature zone increasing the crystal growth rate, and the invention can therefore improve the overwrite erasability of a phase change optical recording medium.
Additionally, the invention is especially effective when used with a non-initialized disc. That is, the invention enables reliable recording of data on non-initialized discs which are available at a low cost, and its industrial advantage is great.